The subject matter disclosed herein generally relates to medical imaging systems and methods. More particularly, the disclosed subject matter relates to systems and methods for correcting errors arising due to a pile-up effect with energy-discriminating, photon-counting detectors when performing material decomposition.
In modern medicine, medical professionals routinely desire to conduct patient imaging examinations to assess the internal condition of a patient in a non-invasive manner. For typical single-energy computed tomography (CT) imaging, the resulting X-ray images are largely a representation of the average density of each analyzed voxel based upon the patient's attenuation of X rays emitted by the X-ray source and detected by the X-ray detector. However, for multi-energy X-ray imaging, a greater amount of imaging data may be gleaned for each voxel, such as an estimate of the type of material in each analyzed voxel. For example, in a dual-energy X-ray imaging system, X-ray spectra with two different energy distributions are employed; higher-energy X-ray photons generally interact substantially less with patient tissue than the lower-energy X-rays. In order to reconstruct multi-energy CT projection data, the underlying physical effects of X-ray interaction with matter are considered, namely, the Compton scattering effects and photoelectric effects, in a process known as material decomposition (MD). Using these techniques, it is possible to identify two or more constituent components in each analyzed voxel.
During multi-energy CT projection data acquisition, a multi-energy X-ray source may be used to generate X-ray spectra having different energy distributions and may be capable of quickly switching from emitting a spectrum having a specific mean energy to emitting a spectrum having a different mean energy, by quickly modifying the peak operating voltage (kVp) of the X-ray tube. Once the X-ray source emits the X-ray spectrum containing a distribution of photon energies, these photons typically pass through a patient or object where they are partially attenuated before reaching an X-ray detector. Typically in such systems, an energy-integrating detector (the energy deposited by the X-ray photons impinging upon the detector cell during an integration period is summed) is used with an X-ray source that rapidly modulates the operating voltage of the X-ray tube. However, another approach for multi-energy imaging is to use an X-ray source emitting a single spectrum and an energy-integrating, photon-counting detector (the detector estimates the energy of each detected photon and tallies the number of photons detected in each of a finite number of energy bins during an integration interval).
An energy-discriminating, photon-counting detector is especially useful in multiple-energy X-ray applications due to this energy-discriminating capability. However, one obstacle impeding its clinical utility is the current count rate capability of this detector technology: the incident photon flux rate required for clinical CT imaging is beyond the counting capability of currently-available energy-discriminating, photon-counting detectors, thus giving rise to a pile-up effect that results in dramatic distortions in the detected signal. The pile-up effect occurs when detector sensor material and electronics cannot keep pace with the incident X-ray flux rate. The energy from multiple photons is deposited during a charge-integration interval in the detector; both the count and energy of detected photons are erroneous if the incident photon flux rate is too high. While methods have been proposed to use the amplitude profile of the detected spectrum to generate a correction for pile-up effect, such methods do not account for the need to perform material decomposition often used for multi-energy imaging. Accordingly, there exists a need for systems and methods that mitigate these limitations.